It is generally known that, if electrical fields are allowed to act on a dispersion system in which fine particles are dispersed in a liquid, the fine particles move (or migrate) in the liquid by Coulomb force (electrostatic force). This phenomenon is referred to as electrophoresis. In recent years, an electrophoretic display device that displays desired information (images) using the electrophoresis draws attention as a new display device.
This electrophoretic display device enjoys reduced power consumption, because it has a display memory property with which a display content is maintained even at the time of stoppage of voltage application. In particular, since the electrophoretic display device performs its display operations using reflected light just like general printed matters, it has such features as a broad viewing angle property and a high-contrast display capability.
As one example of conventional electrophoretic display devices, a patent document 1 discloses an electrophoretic display device that makes use of an electrophoretic dispersion liquid prepared by dispersing two kinds of electrophoretic particles charged with opposite polarities to each other in a liquid phase dispersion medium.
Further, a patent document 2 discloses an electrophoretic display device that makes use of microcapsules, each of which includes an electrophoretic dispersion liquid prepared by dispersing one kind of electrophoretic particles in a liquid-phase dispersion medium, and a shell into which the electrophoretic dispersion liquid is encapsulated.
Furthermore, there has been proposed a combination of the two electrophoretic display devices disclosed in these patent documents, i.e., an electrophoretic display device that makes use of microcapsules, each of which includes an electrophoretic dispersion liquid prepared by dispersing electrophoretic particles for white color display (white particles) and electrophoretic particles for black color display (black particles) in a liquid-phase dispersion medium, the white particles and the black particles being charged with opposite polarities to each other, and a shell into which the electrophoretic dispersion liquid is encapsulated.
In the conventional electrophoretic display devices, an absolute value of a net charge amount of an inner wall of a retention wall (a partitioning wall) or a capsule (the shell) is smaller than an absolute value of a net charge amount of a surface of each of the electrophoretic particles, and charge polarities thereof are opposite to each other.
As a result, if electrical fields act on the electrophoretic particles, they are moved parallel to an application direction of the electrical fields toward an electrode having a charge polarity opposite to the charge polarity of the surfaces of the electrophoretic particles.
With the conventional electrophoretic display devices, a difference in electrophoretic mobility or the like between the electrophoretic particles (the white and black particles) is used in obtaining a gray color having specified gradation which is a color having an intermediate tone (intermediate color) between a white color and a black color.
A specified magnitude of the electrical voltage is applied to between a pair of electrodes for a predetermined time in such a fashion that a perfectly white state is not changed to a perfectly black state or a perfectly black state is not changed to a perfectly white state.
This creates a state that the white particles and the black particles are dispersed or aggregated in a specific region in the liquid-phase dispersion medium. Thus, the gray color is obtained at any rate.
In the conventional electrophoretic display devices, however, it is difficult to obtain a color having a specific intermediate tone such as a gray color or another color having specified gradation.
More specifically, if the electrical voltage is applied to between a pair of electrodes in a perfectly white state or a perfectly black state, the white particles and the black particles are moved from one electrode to the other electrode in a liquid-phase dispersion medium while colliding with each other, respectively.
Further, when the gray color is displayed, the white particles and the black particles exist in a mixed state. This leads to inferior reproducibility and makes it very difficult to obtain a gray color having specified gradation.
Even if the gray color is obtained by stoppage of the application of the electrical voltage to between the pair of electrodes, such a state is unstable and is changed over time.
In other words, since the white particles and the black particles are floating in the liquid-phase dispersion medium, they are moved in the liquid-phase dispersion medium over time. Furthermore, since the white particles and the black particles are electrically charged with the opposite polarities to each other, they are adsorbed together so that a plurality of the white and black particles are aggregated together.
For these reasons, even if the gray color having the specified gradation is obtained, the gray color cannot be maintained as it is and thus a displayed image becomes highly unstable.
Moreover, the mutually adsorbed white and black particles need to be separated prior to providing the next display. To this end, an electrical voltage having higher magnitude is repeatedly applied to between the pair of electrodes while alternately changing its polarity. Otherwise, an additional electrode needs to be provided for that purpose.
This makes a control operation and a structure of the conventional electrophoretic display device complicated, and leads to increased power consumption thereof.
Patent document 1: Japanese Patent No. 800,963
Patent document 2: Japanese Patent No. 2,551,783